Introduction: The assessment of the amount of urine produced by the dose of administered diuretic has been proposed as the main signal of interest in diuretic responsiveness - diuretic efficiency (DE). The main aim of our study is to determine if a low DE is associated with 180-day all-cause mortality (ACM). Methods: During a 3-year period, we retrospectively studied patients with acutely decompensated heart failure (ADHF) and respiratory insufficiency admitted to the emergency room of a tertiary university hospital in Porto, Portugal. A total of 170 patients (age 76.2 ± 10.3 years) were included. The outcome of ACM occurred in 43 (25.3%) patients during the 180-day follow-up period. DE was evaluated for a maximum of 3 h after emergency room admission. The lowest DE was defined as ≤140 mL of diuresis per 40 mg of furosemide equivalents. Results: No significant differences in age, comorbidities, baseline HF symptoms, or disease-modifying medication were found between the lowest and highest DE groups. The lowest DE group had higher blood urea and lower estimated glomerular filtration rate (eGFR) levels (41.3 ± 24.5 vs. 56.7 ± 23.2 mL/min/1.73 m2, p < 0.001). The patients with the lowest DE had significantly higher rates of ACM during the 180-day follow-up, even after adjustment for other clinically relevant variables: hazard ratio (HR) [95% CI] = 2.31 [1.16-4.58], p = 0.016. The lowest diuresis (≤300 mL) and the highest intravenous furosemide dose (>80 mg) alone were not significantly associated with the outcome. After adjustment for N-terminal prohormone of brain natriuretic peptide, the association between the lowest DE and the outcome lost strength (HR [95% CI] = 1.53 [0.75-3.13], p = 0.240). Conclusion: A low DE (≤140 mL/40 mg of furosemide) in the first 3 h after an ADHF episode was associated with increased mid-term mortality rates.